1. Field of the Invention
This invention relates to a method of making a lithographic printing plate by the silver complex diffusion transfer process. In particular, this invention extends the life of the alkaline activator and/or developer solution by reducing the build-up of sludge due to the oxidation of eluted developers from the processed plate.
2. Background of the Art
Photographic precipitates in treatment solutions used in the process of developing silver halide images are well known and understood in the art. Typically the precipitates are due to hard water or silver salt deposition. U.S. Pat. No. 4,310,613 teaches the benefit of quaternary ammonium salts over mercapto-heterocyclic compounds to reduce colored silver sludge in a diffusion transfer process. However, with aminophenol or dihydroxybenzene containing silver halide printing plates processed in a diffusion transfer alkaline activator, a humic acid sludge is formed in developer or activator solutions which limits the useful life of the chemistry.
Ascorbic acid is a well established anti-oxidant, though in photographic applications it is better known as a developer. T. H. James, The Theory of The Photographic Process, 4th. Ed., Macmillan Pub., 1977, p.303-304 provides several references of ascorbic acid reducing silver halide.
EP 498,968 and 531,582 describe ecologically improved graphic arts developers for silver halide emulsions by replacing hydroquinone with at least 0.4 molar ascorbic acid.
WO 93/11456 also states that substituting hydroquinone with ascorbic acid in a rapid access non-diffusing silver halide system developer will reduce sludge build-up. The absence of hydroquinone accounts for the lack of sludge.
As applied to diffusion transfer systems, U.S. Pat. No. 3,730,716 discloses improved stability of developed silver in cine film by the use of .alpha.,.beta.-enediols. U.S. Pat. No. 3,549,364 prevents colored developer oxidation products in a diffusion transfer system by using large quantities of ascorbic acid as an alternative to conventional developers.
Specifically referring to diffusion transfer developers for printing plates, U.S. Pat. No. 3,870,479 claims the use of ascorbic acid and its derivatives in a polyhydroxybenzene/1-pheny-3-pyrazolidone or polyhydroxybenzene/p-aminophenol developer that is essentially free of sulfite. In this particular case, ascorbic acid is used to reduce dot fringe in a wash-off system.
Combinations of chelating agents and ascorbic acid as stable rapid access type developers for non-diffusing silver halide imaging materials are described in U.S. Pat. Nos. 3,887,375; 3,938,997 and 3,942,985.
A review of sequestering agents used in black-and-white photographic developers is given in Research Disclosure 79-18837, and includes aminocarboxylic acids for preventing calcium deposits.
With respect to diffusion transfer systems, U.S. Pat. No. 3,857,710 describes a high contrast monobath processing solution, one of the components being EDTA-tetrasodium salt.
EP 546,598 and 546,599 give examples of diffusion transfer chemistries containing EDTA in a two-sheet duplicating paper process.
Aluminum plates processed in a hydroquinone DTR developer and containing EDTA-tetrasodium salt is also an example in WO 93/11468.
Limitations to using aminocarboxylic acids as sequestering agents due to their being non-biodegradable or accelerating developer oxidation in the presence of trace metal ions are given in U.S. Pat. Nos. 5,200,295 and 4,873,180 respectively.